This invention relates to an improvement in a method for extractive distillation of a hydrocarbon mixture, particularly a C.sub.4 or C.sub.5 hydrocarbon mixture. More specifically, this invention relates to an improvement in an extractive distillation method comprising distilling the hydrocarbon mixture in a polar solvent to withdraw relatively difficultly soluble hydrocarbons from the top of the distillation column and simultaneously withdraw a solvent solution containing relatively easily soluble hydrocarbons from the bottom of the distillation column.
The term "relatively difficultly soluble hydrocarbons" and "relatively easily soluble hydrocarbons" as used in the present invention express the solubility of hydrocarbons in a polar solvent in a comparative manner. For example, when the hydrocarbon mixture is an olefin-diolefin mixture, olefins are the relatively difficultly soluble hydrocarbons and diolefins are the relatively easily soluble hydrocarbons. When the hydrocarbon mixture is a paraffin-olefin mixture, paraffins are the relatively difficultly soluble hydrocarbons and olefins are the relatively easily soluble hydrocarbons. Table 1 summarizes the solubilities of C.sub.4 hydrocarbons in dimethylformamide and N-methylpyrrolidone which are typical polar solvents.
TABLE 1 ______________________________________ Solubility in Boiling Solubility in N--methyl- point dimethylformamide pyrrolidone Components (.degree.C.) (vol/vol/1 atm.) (vol/vol/1 atm.) ______________________________________ iso-Butane -11.7 9.2 (20.degree. C.) 4.87 (40.degree. C.) n-Butane -0.50 16.5 (25.degree. C.) 9.5 (40.degree. C.) Butene-1 -6.26 24.6 (25.degree. C.) 15.6 (40.degree. C.) iso-Butene -6.90 28.0 (25.degree. C.) 15.4 (40.degree. C.) trans-Butene-2 +0.88 35.5 (25.degree. C.) 20.4 (40.degree. C.) cis-Butene-2 +3.72 51.0 (25.degree. C.) 25.1 (40.degree. C.) 1,3-Butadiene -4.41 83.4 (25.degree. C.) 41.5 (40.degree. C.) 1,2-Butadiene +10.85 160.0 (25.degree. C.) 78.0 (40.degree. C.) ______________________________________
C.sub.4 and C.sub.5 hydrocarbon mixtures contain butadiene and isoprene which are important materials for synthetic rubbers and synthetic resins. As olefins, isobutene attracts attention as a material for methyl methacrylate, and n-butene, as a material for maleic anhydride and dehydrogenated butadiene.
Methods for separating these effective ingredients from C.sub.4 and C.sub.5 hydrocarbon mixtures by extractive distillation using polar solvents are disclosed, for example, in Japanese Patent Publication Nos. 17405/1970, 17411/1970 and 41323/1972 and Japanese Laid-Open Patent Publication No. 83421/1981.
Extractive distillation is carried out usually in an apparatus composed of an extractive distillation column and a stripping column. Olefins and/or diolefins which are easily soluble hydrocarbons in the C.sub.4 and C.sub.5 hydrocarbon mixture are withdrawn in the form of a mixture from the bottom of the extractive distillation column and sent to the stripping column where they are separated into the hydrocarbons and the solvent. Generally, the stripping column is operated at a pressure of 0.5 to 5 atmospheres, and the polar solvent substantially free from hydrocarbons is withdrawn at its boiling point under the above pressure from the bottom of the stripping column. Since this solvent is usually at a high temperature of 100.degree. to 200.degree. C., it is returned to the extractive distillation column after its thermal energy is recovered by using it as a heat source for a reboiler of the extractive distillation column and/or a material evaporator (H. Wagner and et al., I. E. C. Vol. 62, No. 4, April 1970, pages 43-48).
The present inventors have variously studied a method for efficiently recovering heat from a solvent which is discharged at a high temperature from the bottom of a stripping column in extractive distillation, and consequently found a method of thoroughly recovering and utilizing the heat of the solvent.